Scientists reverse bad memories by blasting brain cells

Traumatic memories could be switched into pleasant recollections with a flash of light, scientists claim.

Doctors may one day be able to use a new technique to treat soldiers with post-traumatic stress or patients with debilitating phobias.

The method, which has been shown to work in mice, promises the ability to alter memory - a concept that so far has only been the stuff of science-fiction movies.

The treatment works in a similar way to the fictional 'neuralyzer' used by Tommy Lee Jones and Will Smith in the film Men in Black - a bright light that changes memories as the user dictates.

The ability to alter or delete memories has been pursued by increasing numbers of scientists as the full damage caused by traumatic memories becomes clear.

There is a growing understand that recalling an emotional experience, even years later, can bring back the same intense feelings.

Neurologists at the world-leading Massachusetts Institute of Technology in the US have now discovered the brain pathway that link memories to emotions.

They found that a single memory can prompt fear, happiness or excitement depending on the area of the brain it engages.

Their findings, published today in the journal Nature, also demonstrates that the positive or negative emotions of a memory can be reversed.

Study author Dr Susumu Tonegawa, of MIT's Picower Institute for Learning and Memory, said: "In the future, one may be able to develop methods that help people to remember positive memories more strongly than negative ones."

The team demonstrated their technique could work on mice.

One group of male mice were given 'fear' memories with an electric shock to the foot, while others were given 'rewarding' memories by being placed in the same cage as a female mouse.

While the memory was being formed, the researchers monitored which brain nerve cells - called neurons - were activated in the hippocampus and the amygdala, two parts of the brain associated with emotion.

The memory was then reactived using pulses of light to trigger the same neurons, and the mice displayed fearful or happy behaviour, depending which memory had been replayed.

The team then altered the wavelength of the light to trigger neurons in a different part of the mouse's brain, effectively reversing the emotional impact of the memory.

Mice who originally received foot shocks no longer showed fear when recalling that experience.

Conversely, mice that were originally given pleasant memories, were seen to show fear when the new light beam was shone into their eyes.

Roger Redondo, who co-authored the work, said: "Both the hippocampus and the amygdala are considered critical for memory formation.

"We wanted to know whether the memory... was free to associate with positive or negative [emotion] valences or whether it was fixed with respect to emotion.

"We also wanted to know at what point in the circuit the valence is assigned to the engram, in the hippocampus or the amygdala."

The researchers are now investigating whether reactivating pleasant memories has any effect on depression, in the hope of identifying new targets for drugs to treat depression and post-traumatic stress disorder.